1. Field of the Invention
The present invention relates to a fuel injection control system for processing a measured intake air flow rate of an internal combustion engine for an automobile.
2. Description of the Prior Art
A conventional fuel injection control system of this type for an internal combustion engine is as shown in FIG. 1. In FIG. 1, numeral 1 designates an internal combustion engine, numeral 2 an electromagnetic drive type injector (a fuel injection valve) for supplying fuel to the engine 1, numeral 3 a hot air type air flow sensor for detecting air flow rate intaken into the engine, numeral 5 an intake air throttle valve provided at a part of an intake air conduit 6 for regulating intake air flow rate to the engine, numeral 7 a coolant temperature sensor for detecting the temperature of the engine, and numeral 8 a controller for calculating a fuel amount to be supplied to the engine according to an air flow rate signal applied from the sensor 3 to apply a pulse of the width corresponding to a fuel amount request. Numeral 9 designates an igniter for generating a pulse signal at every predetermined rotating angle of the engine, numeral 11 a fuel tank, numeral 12 a fuel pump for pressurizing the fuel, numeral 13 a fuel pressure regulator for maintaining the pressure of the fuel to be supplied to the injector 2 constant, and numeral 14 an exhaust conduit. The controller 3 includes an input interface circuit 80, a microprocessor 81 for processing various input signals to calculate a fuel amount to be supplied to the conduit 6 of the engine 1 in accordance with a program memorized in advance in an ROM 82, thereby controlling a drive signal of the injector 2, the ROM 82, an RAM 83 for temporarily memorizing a data during the calculation of the microprocessor 81, and an output interface circuit 84 for driving the injector 2.
The fuel injection control system thus constructed calculates a fuel amount to be supplied to the engine by the controller 8 according to an intake air flow rate signal detected by the sensor 3 to the engine, provides an engine speed by a rotating pulse frequency produced from the igniter 9, and applies a predetermined pulse width to the injector 2 in synchronization with an ignition pulse. It is necessary to set an air-to-fuel ratio to be required for the engine to a rich side if the engine temperature is low, and the control system corrects to increase the pulse width to be applied to the injector 2 according to the temperature signal from the sensor 7. The system also detects the acceleration of the engine by the change in the opening of the valve 5 to correct the air-to-fuel ratio to the rich side.
Though the hot wire type sensor 3 used to control the fuel in the abovementioned fuel injection control system does not need advantageously atmospheric pressure correcting means due to the detection of the intake air flow rate by weight, the sensor 3 is sensitive to the intake air forced back by the pressure reversing the flow of gas from an exhaust valve toward an intake valve of the engine, taking place when the intake valve and the exhaust valve are opened simultaneously, with the result that the sensor 3 detects the intake air flow rate including the additional intake air thus forced back from the exhaust valve toward the intake valve as the intake air flow rate signal, thereby to generate an output signal of the intake air flow rate slightly larger than the actual intake air flow rate. This additional intake air thus forced back is feasibly generated particularly when the engine is operating in the low speed range with the throttle valve fully opened. As shown in FIG. 2 illustrating the detected intake air flow rate with respect to a time, the waveform of the output of the air flow sensor representing the detected intake air flow rate becomes such that the intake air flow rate might increase due to the additional intake air thus forced back from the exhaust valve toward the intake valve even if the true intake air is not intaken at a time t.sub.R. As a result, the output of the sensor 3 exhibits a considerably larger value than the true value (designated by broken lines in FIG. 3) when the engine is operating in the low speed range with the throttle valve fully opened, as shown in FIG. 3 illustrating the output of the air flow sensor with respect to the opening of the throttle valve. Since an error of the true intake air due to the additional intake air forced back from the exhaust valve toward the intake valve might reach approx. 50% at the maximum depending upon the layout of the engine and the intake air system, this fuel injection control system cannot be utilized in a practical use with this arrangements. In order to compensate this error there has been proposed, as shown in FIG. 4 illustrating the intake air flow rate Q of the internal combustion engine with the throttle valve fully opened with respect to the engine speed, a method of clipping the intake air flow rate at a value (e.g., larger by 10%) slightly larger than the average value b of the true intake air flow rate of the engine, for example, as designated by "MAX" in FIG. 4 by ignoring the output signal a produced from the sensor 3 by setting in advance the maximum intake air flow rate (including an irregularity) to be intaken to the engine in the ROM 82. According to this method, since the clipping value designated by the "MAX" in FIG. 4 might set the maximum intake air flow rate of the engine at a sea level and ambient temperature, the air-to-fuel ratio is largely shifted to a rich side due to a decrease in the actual air density if an automobile with the engine travels on a high ground with low atmospheric air pressure or the engine intakes high temperature air, thereby possibly to cause a high fuel consumption and also to fail to ignite the engine. Further, there might arise a problem that the air-to-fuel ratio is shifted to a lean side if the intake air temperature is low. A method of subtracting a certain value from the actual intake air by judging the waveform of the additional intake air flow rate forced back from the exhaust valve toward the intake valve of the engine has been proposed as a method of correcting an error of the detected intake air flow rate of the air flow sensor 3 due to the additional intake air flow rate forced back from the exhaust valve toward the intake valve. However, the waveform of the intake air flow rate due to the additional intake air flow rate forced back from the exhaust valve toward the intake valve variably depends upon the engine speed and the opening of the throttle valve, and it was difficult to accurately correct the intake air flow rate of the engine.
In the conventional fuel injection control system as described above, the hot wire type air flow sensor 3 has detected larger intake air flow rate than the true value due to the additional intake air flow rate forced back from the exhaust valve toward the intake valve of the engine taking place when the engine rotates in the low speed range with the throttle valve fully opened, and the system has such drawbacks that cannot accordingly properly controls the air-to-fuel ratio in a certain operating range.